This invention relates to a string for tennis racquets, badminton racquets, squash racquets, and the like. More specifically, this invention relates to a racquet string having a core-sheath structure with excellent hitting properties and endurance.
There are three major properties required for racquet strings, namely, hitting properties, endurance and ease of stringing. The hitting properties are those such as rebounding properties, control properties, and spin properties. The hitting properties are also judged by hitting feel and sound, and the like. The endurance of strings is determined by how loose the strings become due to stress relaxation and how long the strings endure abrasion after hits without breaking. Even if a string has two other properties besides the ease of stringing, it remains insufficient and impractical. However, the ratio of importance among these properties (hitting properties: endurance: and ease of stringing is sometimes set at 3:2:1.
Hitting properties are commonly emphasized on the majority of racquet strings, and strings with excellent hitting properties are frequently advertised in the market. As another common trend, strings with small diameters are becoming more popular in the world market than ever before. This tendency is the result of the emphasis on the hitting properties of strings. Several years ago, in the case of tennis racquet strings, 15 gauge strings having 1.41-1.49 mm diameter and 15 L gauge strings of 1.33-1.41 mm diameter were the most marketed string. However, strings currently found in the market are mostly 16 gauge strings of 1.26-1.34 mm diameter.
Furthermore, 16 L gauge strings of 1.22-1.30 mm diameter and 17 gauge strings of 1.16-1.24 mm diameter are also spreading in the market. There have even been user requests for 18 gauge strings of 1.06-1.16 mm diameter. This is because balls rebound better and hitting properties improve as the diameter of the string decreases. The hitting sound and feel of string also improves with smaller diameters, providing a clean and solid hit to players.
A conventional racquet string having a core-sheath structure, for example as disclosed in Published Unexamined Japanese Patent Application (Kokai) No. Sho 60-168857 and Published Examined Japanese Patent Application (Kokoku) No. Hei 1-42069). has a core made of filament fibers and a coating component (sheath) of filament fibers wound around the core or woven outside the core. The use of vinylidene fluoride resin fibers, having a modified cross-section, in racquet strings has also been proposed, in Published Unexamined Japanese Patent Application (Kokai) No. Sho 56-166863 and No. Sho 56-70772.
Strings with excellent hitting properties such as natural gut strings and synthetic strings including polyether ether ketone strings and nylon multifilament strings generally have insufficient endurance. A string of nylon filament fibers of a small diameter loses its endurance, particularly endurance against abrasion, as the diameter becomes smaller.
The endurance of two strings of nylon filament fibers having the same structures but different diameters was tested. The number of hits was counted until the strings of 1.28 mm and of 1.35 mm diameter were broken, and were compared. According to the result, there was about a 40% decline in the number of hits due to the decrease in diameter. In other words, racquet strings found commonly in the global market have good hitting properties but poor endurance. However, demand for durable strings remain strong.
As a durable string, strings made of para-type aramid fibers are commonly sold. Some users are satisfied with these strings, and they are ranked as the most durable strings in the world every year. However, the strings are only several percent ductile and have poorer hitting properties than strings made of gut, etc. Thus, like strings with large diameters, they cannot satisfy the average player. The strings are also expensive.
The ease of stringing is improved by the coating of a smoothing agent such as silicone and wax. However, such treatment is not sufficiently carried out in retail stores.
As explained above, conventional strings do not have all the properties of an ideal racquet stringxe2x80x94hitting properties, endurance and ease of stringingxe2x80x94at the same time. In other words, conventional strings with excellent hitting properties usually have poor endurance, and conventional strings with excellent endurance have poor hitting properties and little ease of stringing.
This invention aims to provide a racquet string which balances and improves three propertiesxe2x80x94hitting properties, endurance and ease of stringingxe2x80x94thus solving the above-noted conventional problems.
More particularly, a racquet string according to the invention comprises of a core surrounded by at least one sheath, having a plurality of sheath fibers wound around said core, and a coating resin uniting the core and sheath fibers into a unitary body. The sheath fibers are helically wound about the core at a high wind angle, i.e., at a wind angle between 25 and 40 degrees relative to the core axis.
Preferably, the sheath comprises a plurality of main sheath fibers and a plurality of non-main sheath fibers. The core, the main sheath fibers, and the coating resin are transparent, and the non-main sheath fibers have a relatively low transparency, such that the non-main fibers produce a multi-helix appearance when the string is observed from the side.
The non-main sheath fibers are preferably made of heat-resistant fibers having a melting point or decomposition temperature of 270xc2x0 C. or higher, and have relatively low transparency.
It is preferable that the core and the main sheath fibers are made of polyamide based synthetic fibers.
It is also preferable that the non-main sheath fibers (heat-resistant fibers) have 10-30% elongation at break.
It is preferable that the non-main sheath fibers (heat-resistant fibers) are at least one type of fiber selected from the group consisting of aromatic polyamide fibers and polyphenylene sulfide fibers. The heat-resistant fibers may also be methaphenylene isophthalic amide fibers.
It is preferable that the string contains the main sheath fibers at 65-90 wt. % and the non-main sheath fibers (heat-resistant fibers) at 10-35 wt. %.
Preferably, the sheath includes 2-6 non-main sheath fibers (heat-resistant fibers).
It is further preferable that the non-main sheath fibers (heat-resistant fibers) are at least one type of fiber selected from the group consisting of multifilament fibers and spun yarns.
Preferably, the non-main sheath fibers (heat-resistant fibers) have 0.4-4.0 twist coefficients which are calculated from the following equation:
t=K{square root over (S)}
where t represents the number of twists (number of turns per 25 mm length of string); K represents the twist coefficient; and S represents yarn counts.
In one embodiment, the string has a single sheath. Alternatively, the string may include two or more sheaths, each containing a plurality of sheath fibers helically wound around the core.
In the preferred embodiment, the above-mentioned racquet string has a core-sheath structure including a multifilament or monofilament core and a plurality of sheath fibers wound around the core, and the core and the sheath fibers are united into one body with a coating resin. The core, the main sheath fibers, and the coating resin are at least essentially transparent. The non-main sheath fibers are heat-resistant fibers having a melting point or decomposition temperature of 270xc2x0 C. or higher, and have relatively low transparency. In this manner, the string has a multi-helical appearance when it is observed lengthwise. And, the string has the three excellent propertiesxe2x80x94hitting properties, endurance and ease of stringingxe2x80x94in balance.
For a better understanding of the invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the drawings accompanying the application.